The invention relates to rotary drill bits for use in drilling or coring deep holes in subsurface formations and, in particular, to methods of manufacturing such bits.
Rotary drill bits of the kind to which the invention relates comprise a bit body having a shank for connection to a drill string and a passage for supplying drilling fluid to the face of the bit. The bit body carries a plurality of cutting elements. Each cutting element may comprise a preform, often circular, having a thin superhard facing layer, which defines the front cutting face of the element, bonded to a less hard backing layer. For example, the superhard facing layer may be formed of polycrystalline diamond or other superhard material, and the backing layer may be formed of cemented tungsten carbide. The two-layer arrangement of the cutting elements provides a degree of self-sharpening since, in use, the less hard backing layer wears away more easily than the harder cutting layer. However, single layer preforms are also known and have the advantage that they may be more thermally stable.
In one type of drill bit of this basic kind, the cutting elements are mounted on the bit body by being bonded, for example, by brazing, to a carrier which may be in the form of a stud of tungsten carbide which is received and located in a socket in the bit body.
The bit body may be machined from steel or may be formed from a tungsten carbide matrix by a powder metallurgy process. In this process a hollow mould is first formed, for example from graphite, in the configuration of the bit body or a part thereof. The mould is packed with powdered material, such as tungsten carbide, which is then infiltrated with a metal binder alloy, such as a copper alloy, in a furnace so as to form a hard matrix. If the cutting elements are of a kind which are not thermally stable at the infiltration temperature, formers, conventionally of graphite, are normally mounted on the interior surface of the mould so as to define on the finished bit body locations where cutting elements may be subsequently located, for example sockets into which the studs on which the cutting elements are mounted may be secured. There may also be mounted on the interior surface of the mold formers which define, in the bit body, sockets to receive nozzles for delivering drilling fluid to the surface of the bit. The nozzle formers may be threaded so that the nozzle sockets are internally threaded to receive threaded nozzles.
Conventionally, the studs on which the cutting elements are mounted are secured within their respective sockets by brazing, press fitting or shrink fitting. While press fitting and shrink fitting are suitable for steel bit bodies where the sockets may be fairly accurately machined, difficulties arise in using such methods with a matrix body. For example, using graphite formers it is found that the dimensions of the sockets provided by the formers cannot be accurately controlled according to the tolerances necessary for press fitting or shrink fitting, with the result that studs may be inadequately secured within the sockets, or attempts to hammer or press a stud into an undersize socket may lead to cracking of the bit body or damage to the cutting structure.
Attempts have been made to overcome this problem by moulding the side walls of the sockets in a manner to give a textured surface so as to increase the permitted tolerances to give a satisfactory interference fit, but such methods have not proved entirely satisfactory. The problem has normally, therefore, been overcome as far as matrix bits are concerned by brazing the studs in the sockets, but it will be appreciated that this adds to the cost of manufacture of the bit. It may also be difficult to remove such brazed studs if it is delivered to repair the bit by replacing worn or damaged cutting structures.
Apart from the above-mentioned problem regarding the accuracy of sockets formed in a matrix bodied bit, difficulties may also arise in removing the formers from the bit body after the infiltration process has been completed.
In order to remove conventional formers, such as graphite formers, from the bit body it is normally necessary to remove them individually by destructive methods, usually involving drilling part of each former out and then mechanically scouring the residue from the socket. These processes are time consuming and expensive in labour costs.
The invention sets out to provide an improved method of forming sockets in a matrix bodied bit in which the above-mentioned problems may be reduced or overcome.